the neurocranium of a brill

Position of the neurocranium

(Image : B. Chanet).

Adult flatfishes, order pleuronectiformes, are peculiar within acanthomorphs, and among vertebrates, by possessing an asymmetrical skull, with the two eyes regrouped to one side (left or right side depending on the species). This page is dedicated to the anatomy of this structure in a flatfish of the family Scophthalmidae: the brill, Scophthalmus rhombus.

In these animals, the skull is strongly deformed in its anterior region during metamorphosis. One of the two eyes, called the migratory eye (oe m) - the right eye in the brill -, migrates over the skull to meet the left eye (oe gauche) on the left side. A bony structure formed by the left lateral ethmoid (eth lat g) and the left frontal (fr g) remains in the skull between the two eyes .

Neurocranium of a brill, left lateral view (Image : B. Chanet).

The posteriormost region are weakly affected by these modifications. In posterior view, only a little deformation of the occipital crest to the right witnesses these events occurring during the growth.

Neurocranium of a brill, occipital view (Image : B. Chanet).

These three images come from the neurocranium of the same specimen (MNHN 1997-410).

Depending on the species, this asymmetry is present on several skeleton structures as well, such as: pectoral and pelvic girdles, vertebrae, splanchnocranium ... Blind-side bones often show differences in size and shape. It is illustrated here — on the website Osteobase — on an other flatfish species: Cyclopsetta querna (Paralichthyidae).

Simplified left lateral view of the head skeleton of a brill (Image : B. Chanet).

The label, with black dots in grey areas, indicates the cartilaginous areas.

in green: neurocranium

in blue+ yellow/red= splanchnocranium

in blue: os enchondral bones

in yellow underlined in red: dermal bones

With some variations, the same organisation is present in all flatfishes:

Neurocranium of a sole (Solea ovata, Soleidae) right lateral view (modified from Chapleau and Keast (1988)).

Contrary to brill and turbot, the two eyes are present on the right side in soleids:

Splanchnocranium of a sole (Solea ovata, soleidés) lateral view (modified from Chapleau and Keast (1988)).
Left, bones of the right side (eyed-side) ; right, bones of the left side (blind-side).
In this species, the asymmetry affects the size and the shape of the bones as well.